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MD3002 - GI Physiology > GI Tract Secretion > Flashcards

GI Tract Secretion Flashcards

1
Q

What is GI function co-ordinated by?

A

integration of neural and hormonal signals

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2
Q

What type of signals stimulate GI function?

A

physical (stretch receptors)

chemical (sights and presence of food)

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3
Q

What are the 3 main phases that regulate GI function?

A
  • cephalic
  • gastric
  • intestinal
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4
Q

Draw a diagram showing the links between the three phases. Highlight both the feed forward and feed backward mechanisms

A
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5
Q

What is the cephalic phase intiated by?

A

taste, thought, smell of food

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6
Q

What is the most important regulator of the cephalic phase and what is it triggered by?

A

Brain and higher iontegrating centres increase vagal activity which intiates the parasympathteic branch of the ANS

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7
Q

What are the effects of the PSNS on the cephalic phase

A
  • increase salivary secretion
  • increase secretion in the stomach
    • HCl
    • Mucus
    • Pepsin
    • Gastrin
  • Stimulation of bile ducts, hepatocytes and pancreatic acinar cells
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8
Q

What occurs as a result of increased secretion in the stomach?

A

increased blood flow to stomach

blood factors (e.g. insulin and glucagon)

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9
Q

What is the gastric phase initiated by?

A

food entering the stomach causing distension and release of gastrin - especially coffee, alcohol, calcium

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10
Q

What are the 4 main inputs into G cells of the stomach during the gastric phase?

A
  • Parasympathetic pathways
  • Decreased acidity to stomach due to buffering of food
  • distension of antrum
  • proteins, peptides and amino acids
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11
Q

What is released by G cells of the stomach and what are their primary targets?

A

Gastrin

  • Gastric motility
  • Parietal cells
  • Trophic maintenance of GI epithelium (cell turn over)
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12
Q

What do parietal cells release that act as a negative feedback mechanism

A

HCl

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13
Q

What is the intestinal phase intiated by?

A

presence of food in the duodenum

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14
Q

What is the requirement for the release of gastrin?

A

If pH >3, duodenal peptides/amino acids cause gastrin release

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15
Q

What is the requirement for gastric inhibition and intestinal stimulation?

A

If pH < 2

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16
Q

What causes release of GIP and CCK?

A

duodenal fats and breakdown products

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17
Q

What causes secretin release

A

acid entering the intestine

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18
Q

What is secretion?

A

Addition of substances (fluids, enzymes, ions etc) into the lumen of the gastrointestinal tract

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19
Q

What are the main producers of secretions?

A
  • Produced by salivary glands (saliva)
  • Cells of the gastric mucosa (gastric secretion)
  • Exocrine cells of the pancreas (pancreatic secretion)
  • Liver (bile)
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20
Q

What are the main characterisitics of saliva?

A
  • High HCO3-
  • High K+
  • Hypotonic
  • α-amylase and lingual lipase
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21
Q

Factors that increase the secretion of saliva

A

PSNS (primary) and SNS

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22
Q

What factors decrease the secretion of saliva?

A
  • sleep
  • dehydration
  • atropine
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23
Q

What are the main functions of saliva?

A
  • Initial digestion of starches and lipids (very little)
  • Dilution and buffering of ingested foods
  • Protection of teeth and gums
  • Lubrication of ingested foods with mucous (mucin)
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24
Q

Describe the location and cell type of sublingual glands

A

deep in floor of mouth, underneath tongue, mostly mucous cells

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25
Q

Describe the location and cell type of subandibular glands

A

under lower edge of mandible, mixed glands containing serous and mucous cells

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26
Q

Describe the location and cell type of parotid glands

A

below ear and over the masseter (jaw muscle), serous cells secreting an aqueous fluid composed of water, ions and enzymes

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27
Q

Describe stage 1 of salivation

A
  • Acini cells secrete 1° secretion (isotonic)
    • Na+, Cl-, K+, HCO3-
    • Amylase and mucin production
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28
Q

Describe stage 2 of salivation

A

Myoepithelial cells stimulated by neural input

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29
Q

Describe stage 3 of salivation

A
  • Duct cells - 2° medication
    • Reabsorb Na+ and Cl- and add K+
    • HCO3- concentration is altered depending on flow rate
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30
Q

Affect of high flow rate on saliva?

A

increased HCO3-

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31
Q

Affect of low flow rate on saliva?

A

more HCO3- extracted

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32
Q

Describe saliva in the mouth in comparison to the plasma?

A

hypotonic (more dilute)

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33
Q

Draw a diagram indicating how salivary secretion is regulated

A
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34
Q

What is the main function of oesophgeal secretion?

A

secretes mucous for lubrication/protection (no real digestive function)

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35
Q

What is the action of simple mucous glands in the main body of the oesphagus?

A

protects against mechanical damage

36
Q

What is the function of compound mucous glands at the gastric end of the oesophagus?

A

protect agaonst chemical damage

37
Q

What are the 4 main types of gastric cells in the stomach and give their exact location

A
  • parietal cells - body
  • chief cells - body
  • G cells - antrum
  • mucous cells - antrum
38
Q

Action of HCl

A

Protein digestion (a bit) – mostly for pepsinogen activation at acid pH, creates pH~2

39
Q

Action of pepsinogen

A

protein digestion (a bit)

40
Q

Action of intrinsic factor

A

vitamin B12 absorption (in the ileum) – really important

41
Q

action of mucus

A

protection and lubrication

42
Q

What are the three main chemicals of gastric secretion

A
  • HCl
  • Pepsinogen
  • Intrinsic Factor
43
Q

What factors increase gastric secretion?

A
  • Gastrin
  • Acetylcholine
  • Histamine
  • Parasympathetic
44
Q

What factors decrease gastric secretion?

A
  • H+ in stomach
  • Chyme in duodenum
  • Somatostatin
  • Atropine
  • Cimetidine
  • Omeprazole
45
Q

Describe HCl secretion on the apical surface

A
  • H+ is secreted into the lumen via the H+-K+ ATPase
  • Cl- follows by diffusion through an apical channel
46
Q

Describe HCl secretion on the basolateral surface

A
  • HCO3- is exchanged for Cl- via the chloride-bicarbonate exchanger (alkaline tide)
  • Eventually HCO3- is secreted back into the GI tract in pancreatic secretions
47
Q

How much does the cephalic phase contribute to secretion?

and what is it triggered by?

A

30% of secretion

smell, taste, chewing, swallowing, conditioned reflex in anticipation of food

48
Q

What is responsible for stimulation of the cephalic phase?

A
  • Direct stimulation of the parietal cells by the vagus
  • Indirect stimulation of the parietal cells by gastrin
    • Vagal gastrin releasing peptide (GRP) stimulates gastrin release from G cells. Gastrin hormone enters the circulation and stimulates parietal cells to release HCl
49
Q

What causes the gastric phase and what percentage does this contribute to secretion?

A

60% secretion

Distension of the stomach and presence of breakdown products of proteins, amino acids and small peptides in the stomach

50
Q

What does distension of the stomach cause?

A

o direct vagal stimulation of parietal cells
o indirect stimulation via gastrin
o local reflexes in the antrum that stimulate gastrin release

51
Q

What are the direct effects of amino acids and small peptides on G cells?

A

stimulates gastrin release

52
Q

What percentage is the intestinal phase of secretion and what is it triggered by?

A

10% of secretion

presence of breakdown products in the duodenum

53
Q

When is HCl secretion inhibited?

A

when HCl is no longer needed to convert pepsinogen to pepsin. This occurs after the chyme moves into the small intestine and the H+ buffering capacity of the food is no longer a factor

54
Q

What is the direct pathway of somatostatin?

A

bindstoreceptorsonparietalcells (and inhibits adenylate cyclase via Gi protein so inhibits HCl release)

55
Q

What is the indirect pathway of somatostatin?

A

inhibitshistaminereleasefrom stomach and gastrin release from G cells so HCl inhibited

56
Q

What is responsible for pepsinogen secretion?

A

secreted by chief mucous cells in the oxyntic glands in response to vagal stimulation

H+ triggers local reflexes which stimulate chief cells to secrete pepsinogen

57
Q

draw a diagram detailing the conversion of pepsinogen to pepsin

A
58
Q

How much does the exocrine pancreas secrete into the duodenum per day?

A

~1L of fluid

59
Q

What is the composition of the fluid secreted into the duodenum by the exocrine pancreas

A

Composed of an aqueous solution containing enzymes and high HCO-3

60
Q

What is the function of HCO3-

A

to neutralise stomach H+

61
Q

Characterisitcs of pancreatic secretion

A
  • High HCO-3 (isotonic)
  • Pancreatic lipase, amylase and proteases
62
Q

Factors that increase pancreatic secretion

A
  • Secretin
  • Cholecytokinin (CCK) (potentiates secretin)
  • Parasympathetic supply
63
Q

What is the innervation of the exocrine pancreas?

A
  • PSNS = vagus stimulates secretion
  • SNS = inhibits secretion
64
Q

How are pancreatic enzymes stored?

A

Stored in condensed zymogen granules until release

65
Q

Where is pancreatic secretion released from and describe the composition of this secretion

A

Aqueous component of secretion released from centroacinar cells and ductal cells

  • Pancreatic fluid is an isotonic fluid containing Na+, K+ Cl-and HCO-3
  • Modification of the (ion) composition by the ductal cells results in a fluid secretion rich in HCO-3
66
Q

Where is the enzymatic component of pancreatic secretion released from and what is its composition?

A
  • Pancreatic amylase and lipases are secreted as active enzymes
  • Pancreatic proteases are secreted in an inactive form and activated in the duodenum
67
Q

Draw a diagram detailing what is released from different cells in pancreatic cells and the ion exchnage that takes place

A
68
Q

What are the three types of pancreatic enzymes?

A
  1. pancreatic amylase
  2. pancreatic lipase
  3. proteolytic
69
Q

action of pancreatic amylase

A

polysaccharide –> disaccharide

much more important than salivary amylase

70
Q

action of pancreatic lipase

A

triglycerides –> monoglycerides and fatty acids

very important source of lipase

71
Q

Action of proteolytic enzymes

A

cleave proteins at different sites –> amino acids and small peptides

72
Q

What are the three main types of pancreatic proteolytic enzymes?

A
  • Tripsinogen –> trypsin
  • Chymotrypsingogen –> Chymotrypsin
  • Procaboxypeptidase –> carboxypeptidase

All inactive to prevent self digestion

73
Q

Draw a diagram showing the activation of proteolytic enzymes

A
74
Q

Describe the regulation of pancreatic secretion in the cephalic phase of digestion

A

mediated by the vagus nerve ( mainly enzymatic secretion)

75
Q

Describe the regulation of pancreatic secretion in the gastric phase of digestion

A

initiated by distension of the stomach and mediated by the vagus nerve ( mainly enzymatic secretion)

76
Q

Describe the regulation of pancreatic secretion in the intestinal phase of digestion

A

accounts for 80% of pancreatic secretion and both enzymatic and aqueous secretions are stimulated

77
Q

Pancreatic Response of acinar cells in the intestinal phase of digestion

A
  • Duodenal I cells secrete CCK in response to the presence of amino acids, small peptides and fatty acids in the intestinal lumen
  • Vagal release of ACh potentiates CCK action
  • Triggers acinar cells to produce enzymes
78
Q

Pancreatic Response of ductal cells in the intestinal phase of digestion

A
  • Secretin released by the S cells of the duodenum is the major stimulus for aqueous rich HCO-3 secretion
  • Secretin release is triggered by the arrival of acidic chyme in the duodenum
  • ACh and CCK potentiate secretin action
  • Triggers ductal cells to produce Na+, K+ Cl- and HCO-3
79
Q

What are the 3 main functions of the gall bladder?

A
  • Stores bile which is continuously produced by the hepatocytes and flows to the gallbladder through the bile ducts
  • Concentrates bile
    • Epithelial cells lining the gallbladder absorb ions and water iso-osmotically
  • Ejects bile
    • Begins ~30 minutes after a meal. The major stimulus for ejection is the release of cholecystokinin from the I cells in the duodenum and jejunum
80
Q

What is bile?

A
  • Essential for the digestion and absorption of lipids
  • Bile salts emulsify lipids to prepare them for digestion and solubilise the products of digestion into ‘packets’ called micelles
  • Mixture of bile salts, bile pigments and cholesterol Bile is not enzymic
81
Q

Describe the regulation of bile secretion

A
  • • In between meals
    • Bile salts recirculated to liver via the enterohepatic system more bile produced
    • Bile stored / concentrated in gall bladder
  • Cephalic phase (just before a meal)
    • Neural (parasym) stimulation via vagus nerve to ↑ bile flow
  • During/after a meal
    • Chyme in duodenum stimulates release of CCK & secretin
82
Q

Action of CCK?

A
  • triggers release of stored bile
  • gallbladder contraction
  • sphincter of oddi relaxation
83
Q

Action of secretin

A
  • triggers bile secretion
  • especially NaHCO3-
84
Q

Draw a diagram indicating the effect of chyme in the duodenum on bile secretion

A
85
Q

Outline the secretions of the small intestine

A
  • Crypts of Lieberkühn
  • Located over the surface of the SI
  • Crypts lie between intestinal villi
  • Crypts and villi covered with two types of cells
    • goblet cells – secrete mucus
    • enterocytes – in crypts secrete water and electrolytes (~1800ml/day)
    • in villi absorb water and electrolytes (along with products of digestion)
  • Exact mechanism for secretion still not known –involves active secretion of Cl- and HCO3- into the crypts at apical membrane. Electrical gradient causes Na+ to be drawn though and together, the osmotically active particles draw water though by osmosis.
  • Regulated largely by distension and tactile or irritative stimuli from chyme
86
Q

Outline the secretions of the large intestine

A
  • Alkaline mucus
    • High[K+]&[HCO3-]
    • No digestive enzymes
    • Functions:
      • protection & lubrication
      • neutralisation of H+ produced by gut bacteria
  • Greatest trigger is distension / mechanical stimulation of walls
    • ↑ secretion triggered by acetyl choline & vasoactive intestinal peptide (VIP)
    • ↓ secreVon triggered by adrenaline & somatostaVn
  • Some parasymp input - Extreme parasymp stimulationexcess mucus - defection (e.g., stress, nervousness)
87
Q

Main stimulus for bile secretion

A
  • secretin
  • CCK (gall bladder contraction: relaxation of sphincter of Oddi)
  • Some central and enteric nerve input

MD3002 - GI Physiology (4 decks)

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